Solar cells and solar modules convert sunlight into electricity. These electronic devices have been traditionally fabricated using silicon (Si) as a light-absorbing, semiconducting material in a relatively expensive production process. To make solar cells more economically viable, solar cell device architectures have been developed that can inexpensively make use of thin-film, light-absorbing semiconductor materials such as copper-indium-gallium-sulfo-di-selenide, Cu(In, Ga)(S, Se)2, also termed CI(G)S(S). This class of solar cells typically has a p-type absorber layer sandwiched between a back electrode layer and an n-type junction partner layer. The back electrode layer is often Mo, while the junction partner is often CdS. A transparent conductive oxide (TCO) such as zinc oxide (ZnOx) is formed on the junction partner layer and is typically used as a transparent electrode.
Although these thin-film based solar cells are known devices, it remains a challenge to cost-effectively produce these cells in sufficient volume with efficiencies greater than or equivalent to their silicon-based counterparts. Because of the difficulties in producing these thin-film cells in volume, many industrial applications of solar cells have continued to be based on traditional silicon-based solar cells with rigid substrates. Certainly, the use of traditional silicon-based solar cells with rigid substrates is a safe, conservative choice based on well understood technology. Traditional solar cells are based on rigid glass substrates using silicon absorber layers and further include rigid glass top layers to provide environmental and structural protection to the underlying silicon based cells.
Drawbacks associated with these rigid cells, however, make them suboptimal choices for applications that require lightweight and/or flexible devices. This may be particularly true for roofing assemblies where it is desirable to have lightweight devices that minimize the load on the roofing support members. From a structural integrity perspective, it is desirable to use materials more robust than glass substrates associated with rigid cells which, in addition to its weight, is also subject to cracking or shattering. From an installation perspective, it is easier to work with a flexible, lightweight substrate that can integrate well with known roofing membranes that are unrolled to cover large areas of a roofing surface.
Furthermore, even if such flexible device were efficiently produced, the types of protective encapsulant materials available today are not well suited for use with such flexible solar cells. Conventional solar cells are usually encapsulated with glass, which is mechanically sturdy (when in a tempered form), impermeable to water vapor and other gases, and suffers no change in optical clarity due to ultraviolet exposure. However, as discussed above, it is also is heavy, expensive, rigid, and fragile. Alternatively, polymer layers are flexible, and many polymers are cheaper than glass, are far lighter, and are not breakable. However, polymers have poorer barrier qualities and may be permeable to small molecule gases such as water and oxygen. Polymers may also be difficult to seal to each other at the edges so that gas intrusion is prevented along the plane of a lamination, and many of them are photochemically reactive and degrade in either flexibility or clarity, or both, by ultraviolet exposure over a period of years or even less. Accordingly, there is a pressing need for improved transparent protective film which can provide improved protection while maintaining flexibility and is suitable for high throughput manufacturing processes.
There remains substantial improvement that can be made to existing photovoltaic roofing assemblies and roofing membranes that can greatly increase their ease of installation, reduce their cost of manufacturing, increase their power density, and improve their durability which should increase market penetration and commercial adoption of such products.